Rockwell International Corp., a major defense and electronics contractor, announced yesterday it has signed an agreement to buy a 68-acre site in western Fairfax County for an office complex that eventually could provide 3,000 jobs.

Rockwell, the $12 billion-a-year builder of the B-1 bomber and sophisticated military satellite systems, said it plans to build an office for its new defense electronics business at Westfields Corporate Center, a major office park on Route 28, south of Dulles International Airport.

The agreement is a major coup for the county, which is faced with an economic slowdown and dissension among officials about how to improve its anti-business image.

The company signed a tentative agreement to buy the land last month, but Rockwell officials said they needed additional time to make sure the site was suitable. The company also had expressed concern about a decision by the County Board of Supervisors to reduce development rights on 14,000 acres of commercial and industrial land.

County officials assured Rockwell that Westfields is exempt from the so-called downzoning law. In addition, the entire downzoning measure has been overturned by a circuit court judge, although the board has decided to appeal his ruling.

Rockwell's decision to come to Fairfax "puts to rest all these myths that the county is anti-business, and I think there are more {businesses} to come," said board Chairman Audrey Moore. "I think it's a sign of the economic stability of this area, the fact that we're attracting businesses now when in fact the rest of the country is in a recession."

"I am delighted," said Supervisor Elaine N. McConnell (R-Springfield), in whose district the office complex will be located. "I think it's just a sign that maybe companies are looking at Fairfax County again. We have such a slow economy. We need this kind of boost right now."

When asked whether Rockwell's decision to come to Fairfax vindicates the Board of Supervisors and its downzoning decision, McConnell said, "I think {Rockwell} came in spite of us. There was no assistance anywhere that I could see."

Rockwell said it does not expect to occupy the 1.5 million-square-foot facility until 1994. The company would not comment further on its decision to locate here. Rockwell's project will be the largest corporate boost for Fairfax since Mobil Corp. moved its world headquarters here in April 1987.

Rockwell, which has headquarters in El Segundo, Calif., employs about 200 people in the Washington area now, mostly in offices in Crystal City. The company had said it wants to be close to Washington because it does a lot of business with the government.